Donald Trump's phone call with Prime Minister Malcolm Turnbull made front pages and news bulletins around Australia and the US, but it led news bulletins in Germany, too, where leading European climatologist Stefan Rahmstorf is based.

"We are all very worried here," Rahmstorf, a professor at the Potsdam Institute for Climate Impact Research, says. "We should assume our worst fears will be realised."

​Such undiplomatic outbursts were standard for Trump before he became US President, but they now carry a manifest menace for the scientific community given his well-chronicled doubts about climate change being real. Among the climate-change deniers he relies on for advice are Scott Pruitt, Trump's pick to head the Environmental Protection Agency, and Rick Perry, his choice as energy secretary.

Some of the possible outcomes scientists fear include the US exiting the Paris global climate agreement that came into force just days before Trump's election win in November and demolishing former president Barack Obama's climate action plan that sought to curb new coal-fired power plants and accelerate the closure of existing ones.

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Scientists fear that under President Donald Trump, the US may exit the Paris global climate agreement.Credit:Getty Images

Pruitt has signalled he wants to roll back regulations, including California's strict auto emissions standards that eventually become global norms. Perry has said "calling carbon dioxide a pollutant is doing a disservice to the country, and I believe a disservice to the world".

Obama transferred a second instalment of $US500 million ($653 million) to the fund in his final days as leader, leaving $US2 billion owing from his original pledge.

The rest of world, including Australia, needs to be prepared to fill any void resulting from any "short-sighted" US pullback, Rahmstorf says. "We have enough information to know we have a serious crisis," he says. "On-going climate change is threatening civilisation."

Nor would Trump be turning against his base if he stymied climate action. Research by the University of New Hampshire found just one in four of Trump voters polled "agree with the scientific consensus that human activities are changing Earth's climate", far less than the wider population and the 90 per cent support from Hillary Clinton voters.

Extreme weather

Climate extremes, particularly at the hot end, are already becoming more common.Credit:Nick Moir

Among the many instances of extreme weather in the past year – 2016, the hottest year on record, had many – the remarkable temperatures monitored over the Arctic have been standouts. This coming week, temperatures over the far north are projected to be as much as 20 degrees above normal.

In Australia, Trump's anti-climate tilt has been echoed with relish by right-wing politicians, such as One Nation and South Australian Liberal senator Cory Bernardi. They have called on the Turnbull government to abandon the Paris agreement in which Australia has pledged to cut 2005-level emissions by as much as 28 per cent by 2030. Australia should also scrap the 2020 Renewable Energy Target in favour of subsidies for new coal-fired power stations, they say.

Beyond the bluster

Scientists fear that, under President Donald Trump, the US may exit the Paris global climate agreement.Credit:Getty Images

But experts at home and abroad caution against anticipating that all of Trump's blustering – such as declaring on Twitter that climate change is a hoax concocted by China – will actually become policy.

Andrew Light, who served as senior adviser to Todd Stern, Obama's special envoy on climate change who led America's Paris negotiations, says it's too early to know Trump's intentions. He points to new US Secretary of State, Rex Tillerson, who has said it's important that the "US maintains its seat" at the Paris table.

Trump's political appointees also continue to hear advice on why it's in America's interests to stay in the agreement.

"The bulk of the people in Paris … will remain in their positions," says Light, who is now a senior fellow at the World Resources Institute. "All of these people know how important it is for US power to stay in."

Those interests include many developing nations who have signed up to Paris because of promises of assistance – such as through the Green Climate Fund – and won't take kindly to reneged offers of help to adapt to a changing climate and investment in renewable energy, Light says.

Also leaning against a radical shift will be the Pentagon, which, over the past decade, has increasingly come to view climate change as a "threat-multiplier" that may tip fragile nations into chaos and pose humanitarian and economic risks.

"The Pentagon as an institution has always been a very moderating influence," Light says. "They can't mess around and look at issues ideologically, or leave it to a guess."

He says climate change, for instance, is widely seen to have played a role in igniting the Syrian civil war that has led to millions of refugees and the emergence of the so-called Islamic State.

Trump will also have no choice but to deal with major nations, such as India and China, who expect the US to keep its emissions promises.

China's President Xi Jinping has spoken out strongly in support of the Paris agreement, which aims to keep global warming to well below 2 degrees compared to pre-industrial times. "The Paris agreement is a milestone in the history of climate governance," Xi told a UN gathering in Geneva last month. "We must ensure this endeavour is not derailed."

The G20 gathering in Germany's Hamburg in July will be one event to watch, assuming Trump hasn't derailed the Paris pact by then. "That's the place where we'll potentially see countries express to President Trump how important climate change is as an area of global diplomacy," Light says.

Nor are critics at home likely to remain idle, with states such as California planning legal action to stall Trump reversals on climate.

"In no way did the elections give Donald Trump a mandate to run roughshod over bedrock environmental laws or all the recent progress made by the Obama administration," said Tiernan Sittenfeld, a senior vice president with the League of Conservation Voters.

"We will go to the mat with our champions in Congress to fight the Trump administration's attacks on our environment and public health, and we are confident that other countries, states, cities, and businesses will continue to act on climate and transition to a clean energy economy."

Money talks

Chris Field, a Stanford scientist who led the IPCC's special report on managing risks of extreme events and disasters, also cautions about assuming the worst case scenario.

"There's clear evidence that the US and all countries benefit from the work of the IPCC, but it is possible that the US will cut its funding," says Field, who is director of Stanford Woods Institute for the Environment.

The US now provides about a third of the global scientific climate panel's funding – for a "few million dollars a year" – and it would be important for other nations to make up any shortfall, Fields says. Scientists volunteering their time make up the biggest contribution.

"The strongest motivation for scientific research on climate change and for assessments to pull that science together is to save lives and save money," Field says. "We're using that knowledge to figure out how to protect economies, communities and eco-systems.

"The essential feature of climate change is to make smart decisions about the future. If you're concerned about protecting [and] keeping this nation safe, climate change is one of the things you need to work on."

US citizens and companies will also suffer "profound disadvantages" if the world becomes less safe. "For a wide range of businesses that are involved in everything from renewable energy to building efficiency, to adaptation solutions, leadership is really going to be key to be growing enterprises in the 21st century," Field says. "Disengagement from the US now forecloses on lots of those opportunities."

Legal hurdles

A threat of a different kind for US businesses may await if Trump pulls up stumps on its global climate obligations, says Tim Stephens, a professor of international law at Sydney University.

Paris is a more robust agreement than the Kyoto protocol that preceded it, and businesses around the world are already moving to cut emissions and refocus investments, he says.

A bigger issue, though, may occur if Trump tries to pull out of the United Nations Framework Convention on Climate Change, which came into force in 1994 and aims to stabilise atmospheric greenhouse gas concentrations and prevent dangerous interruption of the climate.

Exiting the UNFCCC can happen with fewer delays than the Paris accord, but to do so will expose the US "to potential legal liability", Stephens says.

"Under pretty basic principles of international law, they could be held responsible for trans-boundary harm, and there are plenty of precedents to support that," he says."I imagine there'd be pretty clear advice within the State Department that it would be against the US's legal interests to withdraw and denounce the Paris agreement and the UN framework convention."

Stephens says that, while erratic decisions can't be ruled out, much of what is being seen in Washington is political posturing to a domestic audience.

"Diplomatically, the blow-back around the world will be extraordinary were they to do some of these things," he says. "The US name would be mud in so many places where they require co-operation."

Australia has also hit diplomatic blocks "because of our intransigence on climate", including votes in the UN that it has lost because it is viewed as a "pariah", Stephens says. Those diplomatic blocks will become more evident, particularly from at-risk South Pacific nations, should Canberra follow the US on any climate climbdown, he says.

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However, Rahmstorf is not convinced. He worries a US president who is belligerent even to close allies risks creating "an atmosphere of conflict" that won't foster the international co-operation needed to avoid calamitous global warming.

"We can't have everyone fighting for themselves," he says. "We are altogether in one boat and this boat is leaking, and here we are, even starting to quarrel among friends.